1. Field of the Invention
The present invention is concerned with improvements in catalysts useful for the treatment of gases to reduce contaminants contained therein. More specifically, the present invention is concerned with improved catalysts of the type generally referred to as "three-way conversion" or "TWC" catalysts. These TWC catalysts are polyfunctional in that they have the capability of substantially simultaneously catalyzing the oxidation of hydrocarbons and carbon monoxide and the reduction of nitrogen oxides.
2. Background and Prior Art
Catalysts as described above find utility in a number of fields including the treatment of the exhaust from internal combustion engines, such as automobile and other gasoline-fueled engines. Emissions standards for unburned hydrocarbons, carbon monoxide and nitrogen oxides contaminants have been set by various governments and must be met, for example, by new automobiles. In order to meet such standards, so-called catalytic converters containing a TWC catalyst are emplaced in the exhaust gas line of internal combustion engines. The catalysts promote the oxidation by oxygen in the exhaust gas of the unburned hydrocarbons and carbon monoxide and the reduction of nitrogen oxides to nitrogen. If the engine operation is too rich in fuel to provide sufficient oxygen inherently in the exhaust gas, oxygen may be introduced into the exhaust gas as required. The use of separate catalyst beds to promote, respectively, oxidation and reduction, is known and it is also known to use a catalyst system combined in a single bed to substantially simultaneously promote both the oxidation and reduction reactions as described above. It is these types of polyfunctional catalyst systems that are generally referred to as TWC catalysts, as noted above. A great deal of activity has been engendered in the field in an attempt to economically produce catalysts which exhibit good activity and long life in promoting the conversion of hydrocarbons, carbon monoxide and nitrogen oxides, even when contained in very small quantities in a gas stream, to innocuous substances such as carbon dioxide, water and nitrogen. For this purpose, catalysts comprising one or more platinum group metals dispersed upon a high surface area support are well known in the art. The support may comprise a high surface area alumina coating carried on a carrier such as a monolithic carrier comprising a refractory ceramic honeycomb structure, as well known in the art.
Thus, typical catalyst compositions comprise a minor amount of platinum or palladium, preferably including one or more of rhodium, ruthenium and iridium, in particular rhodium, as a platinum group metal component. The platinum group metal component is typically dispersed on a high surface area alumina material which enhances the catalytic activity of the material by dispersing the catalytically active platinum group metal components on a very high surface area support layer. Typically loosely referred to in the art as "gamma alumina" or "activated alumina", such high surface area alumina materials typically exhibit a BET surface area in excess of 60 m.sup.2 /g, often in excess of 80 m.sup.2 /g, e.g., up to about 150 or 200 m.sup.2 /g or more. Such activated alumina is usually a mixture of the gamma and delta phases of alumina, but may also contain substantial amounts of eta, kappa and theta alumina phases.
A common deficiency associated with supported catalyst systems is thermal degradation of the catalyst support from extended exposure to high exhaust gas temperatures of the automotive or other internal combustion engine. In a moving vehicle for example, exhaust temperatures can reach 1000.degree. C., and such elevated temperatures cause the support material to undergo a phase transition with accompanying volume shrinkage, especially in the presence of steam, whereby the catalytic metal becomes occluded in the shrunken support medium with a loss of exposed catalyst surface area and a corresponding decrease in activity. It is a know expedient in the art to stabilize the alumina against such thermal degradation by the use of materials such as zirconia, titania, alkaline earth metal oxides such as baria, calcia or strontia or, most usually, rare earth metal oxides, for example, ceria, lanthana and mixtures of two or more rare earth metal oxides. For example, see U.S. Pat. No. 4,171,288 of Carl D. Keith, et al.
Polyfunctional or three-way conversion catalysts, which serve to substantially simultaneously oxidize hydrocarbons and carbon monoxide and reduce nitrogen oxides, usually require that the ratio of air to fuel ("A/F ratio") introduced into the engine whose exhaust gas is being treated, be at or within a narrow deviation from the stoichiometric ratio. A problem with TWC systems is the adverse effect on catalyst activity caused by the use in automobiles of high A/F ratios which cause greater than stoichiometric oxygen concentration in the exhaust gases. To achieve optimal, substantially simultaneous redox reactions with conventional TWC systems requires the A/F ratio to be in the vicinity of stoichiometric. The use of high A/F ratios in automobile engines improves the fuel economy of the engine, but the presence of excess oxygen in the exhaust, referred to in the art as a "lean exhaust", reduces the activity of platinum group metal catalysts, as platinum is readily sintered at elevated temperatures in a lean exhaust atmosphere, thus reducing the available metal surface area of the catalyst. To achieve optimal simultaneous redox reactions in the exhaust using conventional catalysts, the A/F ratio must be in the vicinity of the stoichiometric A/F since the immediate vicinity of the stoichiometric A/F forms the TWC "window" where the catalyst efficiency is high for the conversion for all three, i.e., hydrocarbon, carbon monoxide and nitrogen oxide, pollutants.
Lean exhaust conditions also have a detrimental effect on the rhodium catalyst. In the Journal of Catalysis, Volume 50, pages 407-418 (December, 1977) in an article entitled, "Surface Interaction in the System Rh/Al.sub.2 O.sub.3 ", the authors report that rhodium interacts strongly with gamma alumina. Under lean exhaust conditions at elevated temperatures, rhodium interacts with and diffuses into the gamma alumina particles. Thus, exposure of TWC systems containing gamma alumina-supported rhodium to lean exhaust conditions results in a reduction in activity believed to be due to a loss of rhodium accessibility to the exhaust system.
The art has devised various methods to improve the catalyst efficiency of Pt/Rh based TWC systems and widen the TWC window. For example, to reduce the rhodium-gamma alumina support interactions, the art has suggested substituting alpha-alumina (U.S. Pat. No. 4,172,047) or zirconia (U.S. Pat. No.4,233,189) as a support material which is not interactive with rhodium. However, alpha-alumina and zirconia are relatively low surface area materials, which is disadvantageous as catalyst activity in such use depends to a certain extent on the surface of the support. During the operation of the vehicle, various catalyst poisons such as lead, zinc and phosphorus are generated from the consumption of fuel and engine oil and deposit non-selectively on the active surfaces of the catalyst metals thereby reducing the available metal surface area of the metal catalyst. As the initial surface area of the TWC material is already low due to the use of the low surface area alpha-alumina or zirconia, the deposition of the poisons may accelerate loss of activity by the TWC system to an unacceptable level.
U.S. Pat. Nos. 3,993,572 and 4,157,316 represent attempts to improve the catalyst efficiency of Pt/Rh based TWC systems by incorporating a variety of metal oxides, e.g., rare earth metal oxides such as ceria and base metal oxides such as nickel oxides, in the TWC system. Thus, in an article entitled "Three Way Catalyst Response To Transients" in Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 288-293 the authors, Schlatter et al report that the operating environment of three-way catalysts is characterized by oscillations of the feed stream composition which occur with a frequency in the order of 1 Hz. It has been suggested that the incorporation of an "oxygen storage" component in the catalyst moderates the effects of the rapid changes between rich and lean exhaust stoichiometries. The authors question the validity of the conventional explanation that the storage component adsorbs excess oxygen during excursions on the lean side of the stoichiometric set point and releases it during subsequent excursions on the rich side. The authors also suggest that the presence of cerium on the rhodium-impregnated spheres in a "fresh" three-way catalyst enhances the performance of the catalyst under transient or oscillating feed stream conditions by increasing either the amount or the stability of the oxidized rhodium species. In a later article, published in the same journal, entitled "Ceria-Promoted Three-Way Catalysts for Auto Emissison Control" Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 274-288, the author, Kim reports that ceria is the best non-noble metal oxide promoter for a typical Pt-Pd-Rh TWC supported on alumina catalyst largely because it enhances the water-gas shift reaction (CO+H.sub.2 O=CO.sub.2 +H.sub.2) and possibly due, in part, to the additional oxygen storage it provides to the TWC.
U.S. Pat. No. 4,539,311 discloses a catalyst for treating motor vehicle exhaust fumes which catalyst is said to have an improved tolerance for lead. A high surface area alumina is impregnated first with a barium moiety, such as an aqueous solution of a barium compound which decomposes to produce barium oxide on firing at over 400.degree. C., and, after such firing, is subsequently impregnated with a dispersion of a platinum group metal moiety such as by soaking the alumina in an aqueous solution of a metal compound which on firing at over 400.degree. C. decomposes to leave behind either the platinum group metal or a compound which converts to the metal when the catalyst is placed in use. The catalyst is made by coating a honeycomb support with alumina incorporating ceria. The dried and calcined alumina coating was then soaked in an aqueous solution of barium nitrate, dried and fired and then soaked in an aqueous solution of chloroplatinic acid, dried and fired. The firing steps were carried out at 550.degree. C.
U.S. Pat. No. 4,294,726 discloses a TWC catalyst composition containing platinum and rhodium obtained by impregnating a gamma alumina carrier material with an aqueous solution of cerium, zirconium and iron salts or mixing the alumina with oxides of, respectively, cerium, zirconium and iron, and then calcining the material at 500.degree. to 700.degree. C. in air after which the material is impregnated with an aqueous solution of a salt of platinum and a salt of rhodium dried and subsequently treated in a hydrogen-containing gas at a temperature of 250.degree.-650.degree. C. The alumina may be thermally stabilized with calcium, strontium, magnesium or barium compounds. The ceria-zirconia-iron oxide treatment is followed by impregnating the treated carrier material with aqueous salts of platinum and rhodium and then calcining the impregnated material.
U.S. Pat. No. 4,504,598 discloses a process for producing a high temperature resistant TWC catalyst. The process includes forming an aqueous slurry of particles of gamma or activated alumina and impregnating the alumina with soluble salts of selected metals including cerium, zirconium, at least one of iron and nickel and at least one of platinum, palladium and rhodium and, optionally, at least one of neodymium, lanthanum, and praseodymium. The impregnated alumina is calcined at 600.degree. C. and then dispersed in water to prepare a slurry which is coated on a honeycomb carrier and dried to obtain a finished catalyst.
European patent application No. 0152052, published Aug. 21, 1985, discloses a monolithic TWC catalyst prepared by impregnating an active alumina powder with a soluble platinum compound, calcining the impregnated powder and then mixing it with a hydrous cerium hydroxide powder, the particle size and water content of which is controlled to assure dispersibility. The mixture is pulverized in a dilute nitric acid solution to prepare a coating slurry which is deposited upon a monolithic support, dried and then calcined. The monlithic support was then impregnated with an aqueous solution of a rhodium salt and dried to provide a finished catalyst.
Japanese patent publication No. 59-127649 published July 23, 1984 (Application Number No. 58/1983-336) dislcoses a TWC and monolithic catalyst having a first base layer of activated alumina supporting platinum, palladium, cerium and lanthanum catalytic elements and a second, upper layer of alumina on which rhodium, iron and lanthanum is dispersed. A first alumina slurry comprising alumina particles impregnated with cerium nitrate and lanthanum nitrate is prepared and coated upon the monolith, dried and calcined at 700.degree. C. The coated monolith was then immersed in an aqueous solution of the platinum compound and dried to form the first layer. Another alumina slurry was prepared with the alumina particles impregnated with lanthanum nitrate and ferric nitrate and calcined and coated onto the monolithic carrier containing the first alumina layer. The monolith was thereafter immersed in an aqueous rhodium compound solution and withdrawn and dried to provide the upper layer.